The only suggestion I would like to make regarding the design is in the default argument for the return value, which tells the implementation not to check for the return value. While it is wildly improbable that any user of the operator traits would actually want to have a trait whose return is actually 'void', it is still theoretically possible. Therefore I think that the default for the return value, which means to not check for the return value, should be an unspecified marker type in the type_traits::detail namespace. It is a convention of all Boost libraries that any library's detail namespace is to be used only by the implementation of that library and not by a user of that library. So using an unspecified marker type in the type traits detail namespace should be fine.
Good news for the return type of operators: I have been able to check for exact void return and have default "dont_care": template< class LHS, class RHS=LHS, class RET=dont_care > has_operator_plus; 1. operator+(A, A) exists and returns void has_operator_plus<A, A>::value -> true (dont check return type) has_operator_plus<A, A, void>::value -> true (check for exact void return type) has_operator_plus<A, A, R>::value -> false (void!=R) 2. operator+(A, A) exists and returns R!=void has_operator_plus<A, A>::value -> true (dont check return type) has_operator_plus<A, A, void>::value -> false (void!=R) has_operator_plus<A, A, R>::value -> true (void!=R) 3. operaot+(A, A) does not exist has_operator_plus<A, A>::value -> false has_operator_plus<A, A, void>::value -> false has_operator_plus<A, A, R>::value -> false I think this is now much better. Frédéric